This invention relates to a process for improving the rheological properties of spray dried, powdered vinyl chloride emulsion polymerizates.
It is known that polymers of vinyl chloride suitable for forming into pastes (plastisols) can be produced by emulsion polymerization employing an alkali salt of fatty acid as polymerization emulsifier, by conducting the polymerization in an alkaline medium, since alkali salts of fatty acids, which have pH values of at least 9.5 in an aqueous medium, are fully effective as emulsifiers only at such alkaline pH values.
By the polymerization in the presence of an alkali salt of a fatty acid as the emulsifier, polymers are obtained having a considerably improved thermal stability, compared to those polymers obtained by the polymerization in the presence of other conventional emulsifiers, such as alkyl sulfates, alkyl and alkylaryl sulfonates (see Table 1). The polymers produced with alkali metal salts of fatty acids, however, form plastisols of unsatisfactorily high viscosities. (See Comparative Examples 1 and 2).
In accordance with a prior process, plastisols of very low viscosities can be obtained from polymers of vinyl chloride produced by emulsion polymerizations employing alkaline salts of carboxylic acids as emulsifiers by reducing the pH of the polymer dispersion to pH values of 4 to 7.5 immediately before or during the spray drying step. The polymerization is conducted at a pH of 9.5-11.5, so that the resultant dispersion remains mechanically stable. See U.S. Pat. No. 4,133,947, whose disclosure is incorporated by reference.
It is an object of this invention to provide a process for improving the rheological properties of vinyl chloride polymers, prepared by emulsion polymerization employing an alkali salt of a fatty acid as the polymerization emulsifier which does not require a pH adjustment step prior to or during spray drying of the polymerizate. Other objects will be apparent to those skilled in the art.